A variety of sensors exist for monitoring the angular velocities of rotating shafts—sensors which are connected to control systems that in one way or another control the operation of such shafts or other machine elements. The typical sensor mounts on a stationary component of a machine opposite a shaft which rotates in that component. The shaft carries a target along its periphery, and the target has the capacity to produce a pulsating signal in the sensor, which signal is monitored by the control system. The target may take the form of alternating ridges and valleys on a gear or spline, or may take the form of a keyway or even pins. It may also assume a magnetic form, consisting of magnetic poles. Sensors assume two basic forms—passive and active. A passive sensor does not require any power from the control system to operate it. But it does require a very small and accurate air gap, and it will not register credible speeds at low angular velocities. A variable reluctance sensor represents one type of passive sensor. An active sensor requires power from the control system and will operate effectively with larger air gaps and at lower speeds. Active sensors are better suited for traction control systems and in assemblies where the air gap is difficult to accurately control. Moreover, some active sensors can sense direction. A Hall-effect sensor represents one type of active sensor.
For a sensor to operate effectively, the air gap between it and the target that it monitors must fall within prescribed tolerances. This requires a good measure of precision during the manufacture of the sensor and the component on which it is installed, or during the installation of the sensor, or both. These exacting demands must also be met during subsequent maintenance procedures which require removal of the sensor and reinstallation. In one type of sensor mounting a hard stop on the sensor seats against a surface on the stationary machine component, thus controlling an air gap with a good measure of precision. The sensor may be removed and reinstalled without too much concern about altering the air gap. However, the sensor and the seat against which its hard stop bears must be manufactured with considerable precision. When a sensor is mounted on a surface that lies perpendicular to or at a substantial angle to the axis of the shaft, no hard stop or seat exists to fix the position of the sensor, while the original installation of such a sensor may be easy, without a hard stop, the reinstallation of the sensor after a maintenance procedure requires exacting efforts.